In mobile communication systems such as for mobile telephones, a cellular scheme is used in which communication is continued over a wide area of combined areas (cells) in which a base station can transmit and receive, as base stations are switched according to the movement of a mobile station. Currently, for example, third generation mobile communication services by code division multiple access (CDMA) are provided. Meanwhile, next generation mobile communication schemes offering faster communication are under investigation.
For example, under the 3rd Generation Partnership Project (3GPP), Long Term Evolution (LTE) and LTE-advanced are under investigation (see, for example, “3GPP TR 36.912 V9.0.0”, September 2009, retrieved from Internet: Oct. 21, 2009 <URL:http://www.3gpp.org/ftp/Specs/archive/36_series/36.423/3 6423-900.zip>). Under LTE-advanced, as a method of improving throughput and coverage (throughput for users at cell edges), the introduction of cooperative communication technology via multiple cells is under investigation. For downlink cooperative communication, cooperative downlink transmission is performed from multiple cells with respect to a given mobile station. For uplink cooperative communication, cooperative uplink communication from multiple cells is performed with respect to the given mobile station.
Under LTE-advanced, joint transmission (JT), fast cell selection (FS), coordinated beam forming (CB), and coordinated scheduling (CS), for example, are under investigation as methods of cooperative downlink transmission.
Further, in an LTE system, base stations communicate with one another to perform handover and control interference. The line for this communication is prescribed as an X2 interface (see, for example, “3GPP TS 36.423 V9.0.0”, September 2009, retrieved from Internet: Oct. 21, 2009 <URL:http://www.3gpp.org/ftp/Specs/archive/36_series/36.912/3 6912-900.zip>). Similar to LTE, under LTE-advanced as well, an interface such as an X2 interface is conceivable.
In cooperative communication, when there are numerous cooperative cells, measurement processing at the mobile station and the amount of information for providing notification of measurement results, from the mobile station to the base station, increase. The amount of control information exchanged among the base stations also increases. Thus, a small number of cells are preliminarily selected as candidate cells. For example, a technique that uses the strength of reference signals received from nearby cells is used as a method of selecting candidate cells. Cells for which the received reference signal strength is strong are selected as candidate cells and when no candidate cell is selected, a cell that is a large source of interference is selected as a candidate cell and the interference is reduced.
Nonetheless, the effectiveness of interference reduction by cooperative communication varies according to factors other than the strength of reception at a cooperative cell. Consequently, in technologies that select cooperative cells based on the strength of reception, interference cannot be sufficiently reduced, arising in a problem that communication quality cannot be improved.